Aspect ratio is the dimensions of an image expressed in ratio format. You determine the aspect ratio by comparing the width and height of an image and then typing it as a width:height ratio (like 3:2 or 4:5).
The aspect ratio of your images is initially determined by the dimensions of your camera's sensor. Because sensor sizes are fixed, it's easy to underestimate the aspect ratio of your images.But it is important that you consider the aspect ratio when taking pictures. The aspect ratio of your camera has compositional effects – and I highly recommend considering that when you're out and about with your camera.
Also while your camera's aspect ratio is technically fixed, many newer digital cameras allow you to change the aspect ratio in the camera menu. You also have the option to post-process an image's aspect ratio, so it's more flexible than you might think.
Different aspect ratios will produce different types of compositions.
For example, square, 1:1 aspect ratio tends to produce very balanced, often limited images.
A 4:5 or 3:2 aspect ratio offers a little more room in the frame.
And the 16:9 aspect ratio gives a lot of room for expansion along the image edges.
Surely, the effect of aspect ratio depends somewhat on the type of scene you are photographing, and some scenes are naturally suited to certain aspect ratios. Therefore, it is very important to carefully consider the aspect ratio before pressing the shutter button; different aspect ratio options can significantly affect composition.
See the image below for more examples of this video editing term.